The interest in high level circuit integration and multi-standard communication systems has been a motivating force behind the development of wideband power amplifiers (PAs). Unfortunately, the usable output power and efficiency of a power amplifier degrade severely as the channel bandwidth and the operation frequencies increase. The requirement of linearity is vital in PAs, even with non-constant amplitude modulation schemes, such as QPSK, OFDM and QAM, because they have fluctuating envelopes and high peak to average ratio. The non-linearity of the power amplifier also causes spectrum regrowth as well as intermodulation distortion (IMD), which leads to inter-symbol interference (ISI) and deteriorates bit error rate (BER) in high speed digital communications. Consequently, wireless communication technologies require high linear power amplifier designs to enable the non-constant modulation techniques. Several prior art schemes have been proposed for linearization of power amplifiers. Backoff from P1dB point, predistortion, feedback and feed forward are the most commonly employed schemes.
“Backoff” biases the power amplifier so that the transmitted signal is much smaller than the maximum signal that the power amplifier is able to handle. Thus, the transmitted signal remains in the linear amplifying region. Backoff, however, is essentially paid for by increased power dissipation of the amplifying devices, more wasted power from the power supply, and the cost to implement the transmitter: all of which are limitations to mobile wireless applications.
The second commonly used method is predistortion. Predistortion consists of the insertion of a set of predistorters in the input of the nonlinear devices to be linearized. Minimization of the total intermodulation power is employed to linearize the bandpass response of the amplifier. In most cases, the predistortion is implemented in a digital technique where the analog input signal is converted to the digital format by an A/D converter. The converted digital signal data is then input to the digital signal processor (DSP) for predistortion in a sample-by-sample manner in accordance with the inverse transfer function of the power amplifier. Finally, the processed data is converted back to an analog signal by a D/A converter to serve as the predistorted input to the power amplifier for amplification. This technique offers high suppression of intermodulation distortion. But the high complexity to implement this technique, the integration issues of the silicon area required and the dissipated power limit the application of this technology.
The third method is the feedback technique which exhibits a lower complexity and offers a reasonable intermodulation distortion suppression. In the feedback method, the distortion of the output signal in both amplitude and phase format is sensed and fed-back negatively to the input so that the combination of the original signal and the feedback signal cancel the intermodulation distortion effect. However, the application of this method is limited due to the inherent stability considerations in implementing any feedback system in radio frequency applications.
A further known method is the technique of feed forward linearization. Feed forward linearization employs a scaled down version of the power amplifier so that the output is subtracted from its input to generate an error signal as the distortion. The error signal is then amplified and combined with the main power amplifier's output to cancel the distortion of the PA. This method does not suffer from stability problems and gives good IMD suppression. However, a major drawback of the feed forward technique is high complexity, leading to high cost and sensitivity to temperature and process variations.
Additional conventional methods employ the structures of transmitters and implement the linearization across the whole transceiver structure: in the baseband stage, the IF stage and the power amplifying stage. Yet these methods also add complexity, cost and additional silicon requirements to the transceiver design and implementation.
Thus, what is needed is an efficient power amplifier design and implementation which maintains stability, linearity and usable output power while suppressing IMD without increasing cost, complexity and silicon area required. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.